It's my understanding that pvc can only be used below ground, to make the transition above would require something else. The size and type would somewhat depend on how far it has to run, the elevation change, and what size are you tapping into. Then, determine the max flow you may want in the trailer and if there would be any lawn watering or other big water usages going on simultaneously. There are codes for regular homes as to what size pipe is needed based on the amount and type of fixtures in the dwelling. Trailers sort of go their own way. If this needs a permit (it does in most places), the inspector would dictate the size based on the local codes and practices.

At a minimum use 3/4" line better would be 1 inch.
I like useing scheduel 80 PVC for water mains, gray in color.
Schedule 80 is okay for use where exposed, so you do not need to switch to copper or some other material.
The pipe itself is fairly inexpensive, the fittings seem to cost almost as much as copper, or perhapse I bought them from the wrong supplier on the last job.
Michael

Without knowing more about the installation, such as "how far is the main from the trailer", "what kind of connection do you have to tie on to", etc., we cannot tell you. In most cases the final connection to the trailer is some flexible material, such as soft copper, garden hose, or similar.